Google today confirmed in a statement that this was a glitch with a particular SDK, and that it remains committed to allowing the stick to play all types of content.

We’re excited to bring more content to Chromecast and would like to support all types of apps, including those for local content. It’s still early days for the Google Cast SDK, which we just released in developer preview for early development and testing only. We expect that the SDK will continue to change before we launch out of developer preview, and want to provide a great experience for users and developers before making the SDK and additional apps more broadly available.

There may of course be suggestions that it was a hasty rethink rather than a glitch; if so, it was done very swiftly.